Anxiety may in part be an inherited condition linked to a specific gene, researchers have suggested.

A team from the National Institute of Mental Health in Bethesda, Maryland, have pinpointed a gene called SLC6A4.

They found people with a certain form of this gene were more prone to anxiety.

However, they admit that the differences were relatively small, and that a single gene in isolation is unlikely to determine whether a person is anxious or not.

Instead, it is likely to be one factor among others.

It is estimated that more than one in ten people suffer from some form of anxiety disorder.

The SLC6A4 gene plays a central role in the transmission of the chemical serotonin around the brain.

Each parent passes either a short or a long version of this gene on to their offspring.

The short version transports serotonin less efficiently, and people with one or two of these tend to show abnormal levels of anxiety.

Brain activity

The researchers tested their theory by measuring the anxiety response in volunteers who were showed pictures of faces with angry or frightened expressions.

They did this by recording levels of activity in an area of the brain called the amygdala, which plays a role in controlling the emotions.

Activity in the amydala was higher for individuals with at least one short version of the SLC6A4 gene.

Researcher Dr Daniel Weinberger told BBC News Online the study was the first to show how a gene contributes to aspects of human emotionality and temperament.

"We have known for a long time that temperament is genetic and that it is present to some degree from very early in life.

"Also we have known that the amygdala is a part of the human brain involved in emotions, particularly the sense of fear and anxiety associated with danger.

"This study shows that one of the factors related to the response of the amygdala to the environment - presumably how quick it is to see dangerousness and how "loudly" it responds - is related to the form of the gene that people inherit."

Dr Weinberger said the next step was to look at other genes and environmental factors that might interact with SLC6A4.

Complex factors

John Fraise, a chartered clinical psychologist at the Adult Psychological Therapy Service in Wakefield, said it was possible that some people had a genetic pre-disposition to anxiety, while others may have become more anxious as a result of life experiences.

He said that the interaction between genes, environment and behaviour was likely to be a complex one.

Even if a gene for anxiety was discovered, a lot of work would be needed to put that knowledge to practical use, he said.

"What I am waiting to find out is not whether a particular gene is involved, but whether things can be engineered to straighten the faulty version of that gene out."